1. Field of the Invention
The present invention relates to a spark plug used in an internal combustion engine, such as an automobile engine.
2. Description of the Related Art
As shown in FIG. 7, a spark plug 200 is used to ignite an air-fuel mixture in an internal combustion engine, such as an automobile gasoline engine. The spark plug 200 is attached to a cylinder head SH of the engine by an attachment screw portion 201a formed on the peripheral surface of a metallic shell 201. In the installed condition, a spark discharge gap g defined by a ground electrode 204 and a center electrode 203 is located within a combustion chamber K and is used to ignite an air-fuel mixture. The electrodes which define the spark discharge gap g are exposed to a combustion gas while the engine is operating, and thus rises to a considerably high temperature.
In recent years, as a power output of an internal combustion engine used in, for example, an automobile increases, an area occupied by an inlet valve and an exhaust valve within the combustion chamber has been increasing. Accordingly, the spark plug must become more compact.
Additionally, due to employment of a supercharger, a turbocharger, or the like, the temperature within the combustion chamber tends to increase more and more.
In order to attain a sufficiently long life of a spark plug under such severe working conditions, heat must be sufficiently radiated from the electrode portions. The heat is radiated from the spark plug in various passages. Particularly, a heat-radiation passage from an insulator 202 to the cylinder head SH via the attachment screw portion 201a exhibits a large heat flow and plays an important role in sufficient heat radiation. In popular spark plugs, the length of the attachment screw portion 201a (screw reach) is as high as 19 to 20 mm even in those of the so-called long reach type. Recently, the screw reach of a spark plug has been lengthened to improve the heat radiation performance of the spark plug.
As is evident from FIG. 7, when the screw reach of the spark plug 200 increases, the overall length of the insulator 202 must increase accordingly. The insulator 202 is generally manufactured by the steps of: compacting a powder of alumina or a similar material by a rubber press or a similar apparatus; lathing or machining the peripheral surface of the compacted body; and sintering the lathed compacted body. However, if the insulator 202 is excessively long, the compacted body may incur eccentric rotation during the lathing process, or the lathed compacted body may bend during the sintering process. These may potentially result in manufacturing deterioration because of unsatisfactory dimensional accuracy. The increase in the length of the insulator 202 leads to a weight increase of the spark plug 200, potentially loosening a caulked portion 201b of the metallic shell 201 because of excessive inertia of the spark plug 200 upon exposure to vibration or impact.
To prevent excessive increase in the overall length of the insulator 202, the length of a projecting portion 202a of the insulator 202 projecting from the tail end of the metallic shell 201 may conceivably be decreased. However, if the length of the projecting portion 202a becomes too short, a discharge may occur between a tail end portion of the center electrode 203 (terminal electrode 205) and the metallic shell 201 along the surface of the projecting portion 202a. This discharge is called a flashover phenomenon. Accordingly, to avoid the flashover phenomenon, the projecting portion 202a cannot be shortened beyond a certain length, which is at least 25 mm according to the common standard in the art.